Most metals oxidize rapidly resulting in thin encapsulating oxide layers on their outer surfaces. This is also the case for solder metals. As solder is heated for reflow soldering, the thin oxide layer remains, thus prohibiting proper wetting of the molten solder and preventing bond formation between device lead and substrate pad.
Soldering requires that the thin oxide film, typically on the order of 100 .ANG. thick, on the surface of both the solder and the substrate be removed or otherwise modified so as to permit wetting. The traditional method of soldering involves the use of a liquid solder flux to remove the oxide. However, chemically active liquid solder fluxes may be corrosive. Thus, cleaning of the flux and flux residues is required in order to prevent corrosion. In addition, liquid solder flux may cause solder splattering or leave voids.
As used herein, the terms "soldering" and "reflow" may be used interchangeably. The significant point is that solder, such as lead-based solder, a tin-based solder, a combination of lead and tin, or other solder is present in the vicinity of a pin of an electronic device such as an integrated circuit chip and the substrate to which it is to be soldered. The soldering or reflow process melts the solder and provides a solid electrical and mechanical connection for the pin to a substrate upon solder solidification.
Some fluxes are viscous and sticky while other fluxes are watery. A conventional liquid flux leaves a residue which may impair the electrical characteristics of the substrate (such as a printed circuit board) or of components. The residue may bear chemicals which are corrosive or may absorb moisture. The moisture attaches to residue salts and may result in electrically conductive paths which ultimately yield electrical shorting. The residue may also make visual solder joint inspection difficult or even impossible.
The removal of some flux residues is expensive and may require the use of environmentally detrimental choloflourocarbons (CFCs). As noted in The New York Times on May 15, 1991, CFCs are known to be detrimental to the Earth's ozone layer and the nation's leading electronics companies are moving rapidly to find alternatives to the use of CFCs.
Some "no-clean" fluxes are available, meaning no post-soldering cleaning operation is required. However, most "no-clean" fluxes still hamper visual inspection of the solder joint and leave unsightly residues on the substrate which may cause long-term reliability problems directly by the constituents of the residue or by enhanced moisture absorption.
Another known system uses a plasma to clean printed circuit boards and components in SF.sub.6 or CF.sub.4 or other fluorinated gases prior to soldering. Such a system requires expensive and slow preprocessing. If the components and printed circuit boards are not stored in an oxygen free atmosphere, the effects of the cleaning are reversed; oxidation of metal surfaces will occur. Any reflow process may be used for boards processed in this fashion but the reflow process must occur in an oxygen-free atmosphere.
Another system employs a small amount (less than 1% by volume) of formic acid. Formic acid at elevated temperatures is known to be an effective reducing agent and can remove some metal oxides. The volume of formic acid is controlled by bubbling nitrogen through a tank containing the liquid chemical. However, this system requires heating the entire substrate with components to greater than 350.degree. C. This system also requires complicated gas curtains and large pieces of equipment and is designed for mass reflow soldering which solders all of the leads on a circuit board simultaneously.
Activators other than formic acid, such as carbonic acid and nitrogen mixed with carbonic acid vapors has also been shown to be effective.
Other known systems pre-process solder and require that the bonding process take place in an expensive and cumbersome vacuum chamber. Also, the use of such a vacuum chamber slows throughput because each time a component to be soldered is placed into the chamber, the chamber must be pumped down to the proper vacuum and this takes time.
Thus, there remains a need for a solder-in-place process that provides localized soldering of individual leads to a substrate. The system used in such a process should be small and inexpensive compared to mass reflow systems. Such a system should preferably use a laser such as a Nd:YAG or other laser to provide both localized soldering and to advantageously eliminate the need to heat the entire system and circuit board to elevated temperatures. Further, the system should eliminate the need for a vacuum chamber and thus should operate at atmospheric or a slight positive pressure.